An ink-jet recording system is a system of recording characters, images, and the like by ejecting small droplets of ink through nozzles, and landing and fixing the ink droplets onto a surface of a recording medium. The ink-jet recording system is widely used in printers at home or offices because of many advantages such as easy full-color printing, low running cost, and less noises in printing. As digital cameras are spread in recent years, there is an increasing demand for printing digital photos taken by the digital cameras by an ink-jet printer, and a recording medium having substantially the same texture and gloss as photographic paper to be used in silver halide photography has been required.
Examples of the recording medium suitably used in photographic printing are glossy plain paper obtained by laminating an ink receiving layer and then a glossy layer on base paper as a substrate; and photo-like paper obtained by laminating a recording layer laminated with an ink receiving layer and a glossy layer, or a recording layer serving as a glossy layer/ink receiving layer on a resin film or resin coated paper as a substrate. As a demand for a recording medium having high gloss and high-quality texture is increasing in recent years, photo-like paper using a substrate having enhanced smoothness has become a mainstream.
A glossy layer when the glossy layer and an ink receiving layer are individually laminate, or a recording layer serving as a glossy layer/ink receiving layer is normally formed by coating an aqueous dispersion solution containing inorganic microparticles such as colloidal silica or alumina sol, and a hydrophilic resin serving as a binder of the inorganic microparticles onto a substrate to impart the recording layer with a function of a void forming agent capable of penetrating/absorbing an ink, and photo-like gloss. As the hydrophilic resin, polyvinyl alcohol is mainly used in view of hydrophilicity with an aqueous ink to be mainly used in ink-jet printing, and bindability with inorganic microparticles.
In glossy plain paper, moistures in the ink are rapidly absorbed in base paper as a substrate through the glossy layer and then the ink receiving layer. Accordingly, water resistance is not significantly important, because there is no likelihood that the glossy layer may hold the moistures for a long time. On the other hand, in photo-like paper, the substrate does not have water absorbability, and the amount of ink per unit area is increased, if multi-color printing is performed to obtain a fine image. As a result, the glossy layer or the glossy layer/ink receiving layer may contain a large amount of moistures immediately after printing. Thus, high water resistance is required for the recording layer. A polyvinyl alcohol-based resin becomes tacky by absorbing moistures. Accordingly, in the case where a glossy layer containing a polyvinyl alcohol-based resin as a binder, or a recording layer serving as an ink receiving layer/glossy layer is formed on a surface of recording media, the recording media are likely to adhere to each other. Preventing the above phenomenon is one of the problems to be solved.
For instance, patent document 1 (JP2002-283697A), and patent document 2 (JP2003-260865A) propose an ink-jet recording medium having an ink receiving layer with enhanced water resistance, wherein the ink-jet recording medium is obtained by forming a recording layer containing polyvinyl alcohol on a substrate, and solidifying the recording layer by a treating solution containing a boron compound. Crosslinking the polyvinyl alcohol by boron is advantageous in enhancing water resistance of PVA, and keeping the ink receiving layer from becoming tacky by absorbing moistures.
There is known a drawback that a recording medium coating solution containing polyvinyl alcohol as a hydrophilic resin, and a boron compound such as boric acid as a crosslinking agent may cause cracks such as streaks or chaps in a coating layer depending on a coating condition, a drying condition after a coating step, or a like condition. To solve this drawback, for instance, patent document 3 (JP2005-199671A) proposes a recording medium formed by simultaneously coating a first ink receiving layer containing a water soluble resin binder having a keto group, and a second ink receiving layer containing a compound having two or more amino groups in a molecule.
The ink-jet recording media proposed in the above patent documents are currently manufactured by simultaneously or sequentially coating a two-part coating solution composed of a solution containing inorganic microparticles and a polyvinyl alcohol-based resin, and a solution (solidifying solution) containing a boron compound. This is because if polyvinyl alcohol, and a boron compound as a crosslinking agent are contained in one coating solution, crosslinking of polyvinyl alcohol may progress depending on an environment such as a temperature, or a condition such as a mixing ratio or a concentration, and the viscosity of the solution may be increased. In a worse case, the coating solution may cause gelation, and may be unusable. The two-part coating solution should be prepared each time the solution is coated, which is cumbersome. Further, since there is no step of uniformly mixing polyvinyl alcohol and a boron compound, uniform crosslinking may not be performed, and properties of the ink-jet recording medium such as water resistance may vary.
The Applicant has proposed, in patent document 4 (JP2006-95825A), an ink-jet recording medium having a coating layer containing a polyvinyl alcohol-based resin having a 1,2-diol unit on a side chain thereof, and inorganic microparticles. Patent document 4 discloses a boron compound as a crosslinking agent which is usable in a two-part coating solution. However, the invention recited in patent document 4 is primarily applied to a glossy layer in glossy plain paper. As mentioned above, since a glossy layer has no need of holding a large amount of aqueous ink, high water resistance is not required for the glossy layer. Further, it was found that the polyvinyl alcohol-based resin recited in Example of patent document 4 did not show sufficiently satisfactory properties, when applied to a photo-like ink-jet recording medium, which is an object of the invention.    [Patent document 1] JP2002-283697A    [Patent document 2] JP2003-260865A    [Patent document 3] JP2005-199671A    [Patent document 4] JP 2006-95825A